1. Field of the Invention
The present invention relates to an optical recording medium and an optical recording method for irradiating the optical recording medium with a laser beam by switching in multi-stages at least one of the duration of time and power of the laser beam in response to the data to be recorded in order to form recording marks of multiple types on the recording layer and thereby record information in multi-levels.
2. Description of the Prior Art
A number of studies have been made on a method for recording multiple pieces of data on each of signals having the same length by switching the depth of a read signal (the degree of modulation of a reflected signal) in multi-stages in contrast to a method for recording data on a conventional optical recording medium by changing the length of a read signal (the length of the modulated portion of a reflected signal) in multi-stages.
This optical recording method, in comparison with a case of recording binary data merely by the presence or absence of pits, makes it possible to record multiple pieces of data in the depth direction. This allows more signals to be assigned to a given length and thus the linear record density to be improved. As a method for switching the depth of a read signal in multi-stages, the power of a laser beam is in general switched in multi-stages to thereby form some recording marks of different types. As its recording medium, suggested currently are those that use holograph or have a multi-layered recording layer.
Incidentally, it is called the multi-level recording to record multiple types of record data different in degree of modulation of reflected signal from each other.
On the other hand, these optical methods for multi-level recording had a problem of providing signals that were degraded in quality at the time of reading as the power of a laser beam for use in recording increased, that is, as a reflected signal to be formed was increased in depth. The reason for this has not been made clear until now, however, the inventor predicts that this is conceivably caused by an increase in area of a recording mark (recording mark area) resulting from an increase in the laser power. For example, suppose that a conventional method was used to shorten recording marks to provide a recording medium with an amount of information recorded at a high density, in which the power of a laser beam was switched in multi-stages to perform multi-level recording. In this case, the quality of signals was significantly degraded, resulting in making no use of the merit of the multi-level recording. That is, to employ the multi-level recording, the recording marks had to be spaced widely apart from each other so that data could be positively detected to some extent even when the quality of signals were degraded. For this reason, the conventional method had a problem that recording marks were provided at a high density with difficulty, thus resulting in mutually contradictory circumstances.
The concept of a conventional optical recording method for performing multi-level recording by switching the power of a laser beam in stages is based on the premise that the length of a recording mark is greater than the diameter of a converging beam (beam waist) available at the time of recording. That is, the concept aims to realize multi-level recording by modulating the optical reflectivity of the recording mark itself in multi-stages and then directly reading the reflectively.
In general, the diameter of a converging beam is expressed by Kxcex/NA (where K is a constant, xcex is the wavelength of the laser, and NA is the numerical aperture of the lens). In general, xcex=780 nm and NA=0.45 are employed in pickups for use with CDs with the converging beam being about 1.6 xcexcm in diameter. In this case, with the recording mark being around 1.6 xcexcm in length, the aforementioned problem of signal degradation become apparent, making it difficult to perform multi-level recording in five stages or more. According to a close study made by the inventor, this was presumably in part because of the fact that the prior art was based on the precondition that the length of the recording mark was greater than the diameter of the converging beam (beam waist) available at the time of recording, then modulated the optical reflectivity of the recording mark itself in multi-stages, and directly read the reflectivity to thereby realize multi-level recording.
The problems mentioned above are conceivably resulted from every factor, intertwined in a complicated manner, such as the power setting of the laser beam or the characteristics of the recording medium. However, as far as known to the inventor, the causes of the problem have not yet been made clear until now. Thus, the real picture is that the multi-level recording of a high density as well as its recording method have not yet been accomplished.
The present invention has been developed in view of the aforementioned problems. It is therefore the object of the present invention to suggest a new multi-level recording method by setting the characteristics of a recording medium to a predetermined condition, thereby accomplishing the multi-level recording of a high density.
The inventor has made intensive studies on optical recording media, and found a method for assuming virtual recording cells of a given size on the recording layer, where recording marks of different sizes were formed, to modulate the reflectivity of the entire virtual recording cells in multi-stages for multi-stage recording. The inventor confirmed that the method made it possible to perform multi-level recording of high densities in five stages or more on an optical recording medium. That is, the invention described below makes it possible to achieve the aforementioned object.
(1) An optical recording medium in which on an optical transparent substrate having predetermined grooves, a recording layer is formed at least to cover the grooves and irradiated with a laser beam to form recording marks at least on the recording layer to thereby record information on the recording layer, the optical recording medium wherein on the recording layer, contiguously defined along a feed direction of the laser irradiation are virtual recording cells which have a given unit length in the feed direction along the groove and a given unit width in the direction orthogonal thereto; the groove width W is set so that 0.20xc3x97(xcex/NA) less than W less than 0.50xc3x97(xcex/NA), where xcex (nm) is the wavelength of the laser beam and NA is the numerical aperture of the objective lens for the laser beam in a irradiation optical system; at least either the irradiation time or irradiation power is set in five stages or more to radiate the virtual recording cell with the laser beam, thereby allowing for forming recording marks having different sizes of five types or more on the virtual recording cell; and optical reflectivity is modulated in accordance with the area ratio of the recording mark to the virtual recording cell, thereby allowing for recording information in multi-levels.
(2) The optical recording medium according to (1) in the foregoing, characterized in that the groove width W is set so that 0.25xc3x97(xcex/NA) less than W less than 0.45xc3x97(xcex/NA).
(3) The optical recording medium according to (1) or (2) in the foregoing, characterized in that a plurality of the grooves are formed generally parallel to each other, and the pitch P between the grooves adjacent to each other is set so that 0.65xc3x97(xcex/NA) less than P, more preferably 0.7xc3x97(xcex/NA) less than P less than 1.2xc3x97(xcex/NA).
(4) The optical recording medium according to (1), (2), or (3) in the foregoing, characterized in that the recording layer is adapted to contain an organic dye.
(5) The optical recording medium according to any one of (1) to (4) in the foregoing, characterized in that the recording layer is adapted to contain a cyanine base dye.
(6) An optical recording medium in which on an optical transparent substrate having predetermined grooves, a recording layer mainly including phthalocyanine dye is formed at least to cover the grooves and irradiated with a laser beam to form recording marks at least on the recording layer to thereby record information on the recording layer, the optical recording medium wherein on the recording layer, contiguously defined along a feed direction of the laser irradiation are virtual recording cells which have a given unit length in the feed direction along the groove and a given unit width in the direction orthogonal thereto; the groove width W is set so that 0.25xc3x97(xcex/NA) less than W less than 0.55xc3x97(xcex/NA), where xcex (nm) is the wavelength of the laser beam and NA is the numerical aperture of the objective lens for the laser beam in a irradiation optical system; at least either the irradiation time or irradiation power is set in five stages or more to radiate the virtual recording cell with the laser beam, thereby allowing for forming recording marks having different sizes of five types or more on the virtual recording cell; and optical reflectivity is modulated in accordance with the area ratio of the recording mark to the virtual recording cell, thereby allowing for recording information in multi-levels.
(7) The optical recording medium according to (6), wherein the groove width W is set so that 0.30xc3x97(xcex/NA) less than W less than 0.50xc3x97(xcex/NA).
(8) The optical recording medium according to (6) or (7), wherein a plurality of the grooves are formed generally parallel to each other, and the pitch P between the grooves adjacent to each other is set so that 0.65xc3x97(xcex/NA) less than P, more preferably 0.7xc3x97(xcex/NA) less than P less than 1.2xc3x97(xcex/NA).
(9) An optical recording medium with an optical transparent substrate having predetermined grooves and a recording layer formed at least to cover the grooves, the recording layer being irradiated with a laser beam to form recording marks and record information at least on the recording layer, the optical recording medium wherein on the recording layer, virtual recording cells are contiguously defined along a feed direction of the laser irradiation for performing recording and reading operations on the groove, the virtual recording cell having a given unit length in the feed direction and a given unit width in a direction perpendicular thereto, a minimum film thickness of the recording layer with respect to a bottom surface of the groove is set to be smaller than a depth of the groove on the optical transparent substrate, and at least either an irradiation time or an irradiation power is set in five stages or more to irradiate the virtual recording cell with the laser beam, thereby allowing for forming recording marks having different sizes of five types or more on the virtual recording cell, and an entire optical reflectivity of the virtual recording cell is modulated based on an area ratio of the recording mark to the virtual recording cell, thereby allowing for recording information in multi-levels.
(10) The optical recording medium according to (9), Wherein the minimum film thickness M of the recording layer with respect to the bottom surface of the groove is set in terms of the groove depth F on the optical transparent substrate, so that 0.2xc3x97F less than M less than 1.0xc3x97F.
(11) The optical recording medium according to (9) or (10), wherein the minimum thickness M of the recording layer with respect to the bottom surface of the groove is set in terms of the groove depth F on the optical transparent substrate, so that 0.3xc3x97F less than M less than 0.8xc3x97F.
(12) The optical recording medium according to (9), (10), or (11), wherein the minimum thickness M of the recording layer with respect to the bottom surface of the groove is set in terms of the groove depth F on the optical transparent substrate, so that 0.4xc3x97F less than M less than 0.6xc3x97F.
(13) The optical recording medium according to any one of (9) to (12), wherein the recording layer contains an organic dye.
(14) The optical recording medium according to (13), wherein the organic dye contained in the recording layer comprises a cyanine base dye.
(15) An optical recording medium with a recording layer formed on an optical transparent substrate and irradiated with a laser beam to form recording marks and record information on the recording layer, wherein within a recording mark formation region on the optical recording medium, a plurality of virtual recording cells are contiguously defined in a direction of movement relative to the laser beam, the virtual recording cell having a predetermined unit length and a predetermined unit width in a direction perpendicular thereto, a characteristic of the virtual recording cell is set so that (Xxe2x88x92Y)/X is 0.3 or more, preferably 0.4 or more, where X(%) is an initial reflectivity of the virtual recording cell without being irradiated with the laser beam and Y(%) is a minimum reflectivity limit for a maximum recording mark formed by being irradiated with the laser beam, and at least either an irradiation time or an irradiation power of the laser beam is modulated in five stages or more to irradiate the virtual recording cell with the laser beam, thereby allowing for multi-level recording.
(16) The optical recording medium according to (15), wherein when the recording marks are formed by switching at least either the irradiation time or the irradiation power of the laser beam in five stages or more to perform multi-level recording, the characteristic of the virtual recording cell is set so that a variation in reflectivity of all recording mark formation regions at each stage falls within 5%, preferably within 3%.
(17) The optical recording medium according to (1) or (2), wherein the characteristic of the virtual recording cell is set so that (Xxe2x88x92Y)/Xxe2x89xa60.9.
(18) An optical recording method for an optical recording medium with a recording layer formed on an optical transparent substrate and irradiated with a laser beam to form recording marks and record information on the recording layer, wherein within a recording mark formation region on the optical recording medium, a plurality of virtual recording cells are contiguously defined in a direction of movement relative to the laser beam, the virtual recording cells having a predetermined unit length and a predetermined unit width in a direction perpendicular thereto, a characteristic of the virtual recording cell is set so that (Xxe2x88x92Y)/X is 0.3 or more, preferably 0.4 or more, where X(%) is an initial reflectivity of the virtual recording cell without being irradiated with the laser beam and Y(%) is a minimum reflectivity limit for a maximum recording mark formed by being irradiated with the laser beam, and at least either an irradiation time or an irradiation power of the laser beam is modulated in five stages or more to irradiate the virtual recording cell with the laser beam, thereby allowing for multi-level recording.
(19) The optical recording method according to (18), wherein when the recording marks are formed by switching at least either the irradiation time or the irradiation power of the laser beam in five stages or more to perform multi-level recording, the characteristic of the virtual recording cell is set so that a variation in reflectivity of all recording mark formation regions at each stage falls within 5%, preferably within 3%.
(20) The optical recording method according to (18) or (19), wherein the characteristic of the virtual recording cell is set so that (Xxe2x88x92Y)/Xxe2x89xa60.9.
(21) An optical recording medium in which on an optical transparent substrate having predetermined grooves, a recording layer is formed at least to cover the grooves and irradiated with a laser beam to form recording marks at least on the recording layer to thereby record information on the recording layer, the optical recording medium wherein on the recording layer, contiguously defined along a feed direction of the laser irradiation are virtual recording cells which have a given unit length in the feed direction along the groove and a given unit width in the direction orthogonal thereto; the groove width W is set so that 0.20xc3x97(xcex/NA) less than W less than 0.50xc3x97(xcex/NA), where xcex (nm) is the wavelength of the laser beam and NA is the numerical aperture of the objective lens for the laser beam in a irradiation optical system; a minimum thickness of the recording layer with respect to a bottom surface of the groove is set to be smaller than a depth of the groove on the optical transparent substrate at least either the irradiation time or irradiation power is set in five stages or more to radiate the virtual recording cell with the laser beam, thereby allowing for forming recording marks having different sizes of five types or more on the virtual recording cell; and optical reflectivity is modulated in accordance with the area ratio of the recording mark to the virtual recording cell, thereby allowing for recording information in multi-levels.
(22) The optical recording medium according to (21), wherein a characteristic of the virtual recording cell is set so that (Xxe2x88x92Y)/X is 0.3 or more, preferably 0.4 or more, where X(%) is an initial reflectivity of the virtual recording cell without being irradiated with the laser beam and Y(%) is a minimum reflectivity limit for a maximum recording mark formed by being irradiated with the laser beam.
(23) An optical recording medium with an optical transparent substrate having predetermined grooves and a recording layer formed at least to cover the grooves, the recording layer being irradiated with a laser beam to form recording marks and record information at least on the recording layer, the optical recording medium wherein on the recording layer, virtual recording cells are contiguously defined along a feed direction of the laser irradiation for performing recording and reading operations on the groove, the virtual recording cell having a given unit length in the feed direction and a given unit width in a direction perpendicular thereto, a minimum thickness of the recording layer with respect to a bottom surface of the groove is set to be smaller than a depth of the groove on the optical transparent substrate, a characteristic of the virtual recording cell is set so that (Xxe2x88x92Y)/X is 0.3 or more, preferably 0.4 or more, where X(%) is an initial reflectivity of the virtual recording cell without being irradiated with the laser beam and Y(%) is a minimum reflectivity limit for a maximum recording mark formed by being irradiated with the laser beam, and at least either an irradiation time or an irradiation power is set in five stages or more to irradiate the virtual recording cell with the laser beam, thereby allowing for forming recording marks having different sizes of five types or more on the virtual recording cell, and an entire optical reflectivity of the virtual recording cell is modulated based on an area ratio of the recording mark to the virtual recording cell, thereby allowing for recording information in multi-levels.
(24) An optical recording method for an optical recording medium with a recording layer formed on an optical transparent substrate and irradiated with a laser beam to form recording marks and record information on the recording layer, wherein within a recording mark formation region on the optical recording medium, a plurality of virtual recording cells are contiguously defined in a direction of movement relative to the laser beam, the virtual recording cells having a predetermined unit length and a predetermined unit width in a direction perpendicular thereto, a characteristic of the virtual recording cell is set so that (Xxe2x88x92Y)/X is 0.3 or more, preferably 0.4 or more, where X(%) is an initial reflectivity of the virtual recording cell without being irradiated with the laser beam and Y(%) is a minimum reflectivity limit for a maximum recording mark formed by being irradiated with the laser beam, the groove width W is set so that 0.20xc3x97(xcex/NA) less than W less than 0.50xc3x97(xcex/NA), where xcex (nm) is the wavelength of the laser beam and NA is the numerical aperture of the objective lens for the laser beam in a irradiation optical system, and at least either an irradiation time or an irradiation power of the laser beam is modulated in five stages or more to irradiate the virtual recording cell with the laser beam, thereby allowing for multi-level recording.
(25) The optical recording method according to (24), wherein a minimum thickness of the recording layer with respect to a bottom surface of the groove is set to be smaller than a depth of the groove on the optical transparent substrate.
The inventor found that a new modulating method employing the occupation ratio of the recording mark to the virtual recording cell enables multi-level recording. As a result, the recording density can be dramatically improved.
However, it was also found that forming recording marks only by modulating the irradiation time or irradiation power would not make it possible in some cases to positively read the recording marks.
One of the reasons for this was conceivably that the multi-level recording which requires the setting of optical reflectivity in the virtual recording cell in multi-stages with high accuracy is particularly subject to the effect of the structure of the optical recording medium. More specifically, a number of studies made by the inventor showed that in the case of the present invention where recording marks were formed in multi-stages, recording marks smaller than the spot of the laser beam could be formed, thereby making it possible to set a narrower groove width in comparison with the prior art, while an error in optical reflectivity of each stage suddenly increased in the virtual recording cell when the groove width W was less than or equal to 0.20xc3x97(xcex/NA)(0.25xc3x97(xcex/NA) or less in the case where the recording layer was made of a phthalocyanine base dye). One of the reasons for this is presumably that the recording mark is restricted by the width of grooves so as not to be able to grow up to a predetermined size, thereby causing the degree of modulation to degrade.
Furthermore, for an uncertain reason, it was also found that such a phenomenon occurred that actual reflectivity levels were varied among a plurality of virtual recording cells that were expected to have the same reflectivity level as the groove width became narrower (i.e., reproducing repeatability was degraded), thereby significantly reducing signal quality (degrading error values).
On the contrary, cross-talk (a phenomenon of reading the recording mark in an adjacent groove at the same time in conjunction with the recording mark that should be originally read) may readily occur due to the effect of the recording mark in an adjacent groove when the groove width W is greater than or equal to 0.50xc3x97(xcex/NA)(0.55xc3x97(xcex/NA) or more in the case where the recording layer was made of a phthalocyanine base dye) with the track pitch remaining unchanged. To prevent this, it is necessary to increase the spacing between grooves (hereinafter referred to as the track pitch), however, an unnecessarily increase in track pitch causes the recording density to reduce.
Furthermore, for an uncertain reason, it has also been found that when relatively large recording marks are formed to provide the virtual recording cell with a low reflectivity level, the reflectivity varies significantly. This would lead to a degradation in signal quality such as a degradation in error value.
That is, it is necessary not only to set the groove width in response to the type of the recording layer or the like as in the current binary recording method but also to set the groove width suitable for the multi-level recording in order to perform the multi-level recording. Failure to set the groove width as such would result in degradation in signal quality characteristic of the multi-level.
This tendency, although currently under analysis, depends also on the linear velocity at the time of recording. It is thus necessary to select the groove width in response to the recording linear velocity. However, since the applicable range (margin) of grooves tends to become narrower as the recording linear velocity becomes higher (e.g., (8 times or more to about 20 times the standard linear velocity 1.2 m/s such as of a CD-R), such design as based on the assumption of recording at certain high velocities is conceivably able to cover the range of recording at low velocities.
This has been confirmed to be noticeable and useful particularly when the optical reflectivity is modulated in five stages or more to record information (in comparison with cases where the number of stages is less than this case). Incidentally, the aforementioned present invention may be added by the following configuration.
An optical recording medium characterized in that a groove for guiding a laser beam is provided along the recording layer, the virtual recording cell is set within the groove, and the unit width is generally equal to the width of the groove.
An optical recording medium characterized in that any identifying information indicative of multi-level recording, binary recording, and bar-code recording, which indicates that the optical recording medium is a multi-level recording medium, at the virtual recording cell or any position of the optical recording medium.
An optical recording medium characterized in that a groove for guiding a laser beam is provided along the recording layer, where the groove is interrupted in part.
Incidentally, what is meant here by the groove width means the width of the groove at the position that generally bisects the distance between the highest point of the land sandwiched by grooves and the maximum groove depth obtained by the lowest point of the groove bottom portion, being a value that is called a half-depth width in general. This definition derives from the consideration of a case where the groove (or the land) is trapezoidal in cross section. In addition, attention was also focused on the possibility that the desirable result would be provided by employing a shape or configuration that is different from that of the optical recording medium that was applied to conventional binary recording, as the configuration of an optical recording medium applied to the multi-level recording.
In the binary recording, a larger xe2x80x9cdifferencexe2x80x9d in optical reflectivity upon reading between binaries and a more distinct boundary (mark edge) between the binaries are desirable. To realize these, the thickness of the record film was set so as to be larger with respect to the depth of the groove (e.g., at 120% to 150% of the groove depth).
This allows the recording layer on the groove side to become continuous to the recording layer on the land side to some extent. Irradiating a region corresponding to the groove with a laser beam at a predetermined irradiation power would make it possible to extend (expand) the recording mark instantly towards the land. As a result, the amount of an absolute variation in optical reflectivity was increased and the mark edge was made distinct.
However, in the case of the multi-level recording technique according to the present invention for obtaining optical reflectivity in multi-stages by modulating the occupation ratio of the recording mark to the virtual recording cell, it is required to accommodate the optical reflectivity of each virtual recording cell within the allowable tolerance of each level. That is, conversely, expanding the recording mark freely as in the binary recording makes it difficult to control the optical reflectivity of each virtual recording cell.
On the aforementioned presumption, the present invention is adapted such that the minimum film thickness of the recording layer is set to be smaller than the depth of the groove with respect to the bottom surface of the groove. As a result, the recording layer depressed with respect to the groove allows for making the recording layer thinner in thickness (including the case of zero thickness) at the edge portion formed at the boundary between the groove and the land. This prevents the recording mark formed in the groove region from expanding towards the land and the recording mark formed on the land side from expanding towards the groove.
As a result, the recording mark is prevented from expanding unnecessarily with respect to the virtual recording cell in the width direction, making it possible to set the occupation ratio of the recording mark to the virtual recording cell with high accuracy and actually achieving the multi-level recording in five stages or more.
Incidentally, the aforementioned present invention may be added by the following structure.
An optical recording medium characterized in that the virtual recording cell is set to the region corresponding to the groove, and the unit width is generally equal to the width of the groove.
An optical recording medium characterized in that identifying information indicating that the optical recording medium is a multi-level recording medium is recorded on the virtual recording cell.
An optical recording medium characterized in that the groove is interrupted in part.
The inventor paid attention to both the variation characteristic in reflectivity of the virtual recording cell in the optical recording medium and the multi-level recording technique. In particular, the inventor paid attention to the dynamic range of reflectivity and/or the variation in reflectivity at the same recording level, and found that a better multi-level recording could be achieved when the former was greater than or equal to a certain value and/or the latter was within the range of predetermined values.
According to the analysis performed by the inventor, it was found that when a recording mark reduces reflectivity, some extent of the difference (dynamic range) is required between the reflectivity of a virtual recording cell having a maximum recording mark formed thereon and the reflectivity of a virtual recording cell having no recording mark formed thereon, in order to perform the multi-level recording of the virtual recording cell with a variation in reflectivity in five stages or more, wherein in the case of multi-level recording, the value of the dynamic range had adverse effect on recording and reading operations since the reflectivity of respective signals interferes with each other unless the reflectivity levels of the signals are spaced by a certain amount from each other.
In this context, various studies were made to properly perform recording and reading operations that were not affected by the interference between signals. As a result, it was found that the dynamic range had to be set to a value greater than or equal to a certain value by optimizing various structures such as the shape of the groove and the thickness of the recording layer particularly in order to perform a multi-level recording in five stages or more.
As the result of actual measurements, a multi-level recording was performed properly in five stages or more with the dynamic range being 30% or more, preferably 40% or more. In particular, the effect was remarkable when the length set to the virtual recording cell was shorter than the diameter of a beam.
Furthermore, within the same recording area, the variation in reflectivity has to be made less than that of the conventional binary recording at each stage of the multi-level recording.
The variation in reflectivity within the same recording area can be set by the number of stages of the multi-level recording, however, it was found that the aforementioned variation was preferably within 5% in the case of the multi-level recording in five stages or more. More preferably, the variation is 3% or less. In the case of the multi-level recording in five stages or more, a greater dynamic range is preferable, however, the upper limit is as shown by the aforementioned range since an excessive dynamic range causes distortion of signals to occur.